The present invention relates to the manufacture of tubes from spheroidal graphite cast-iron by centrifugal casting and more particularly to a thermal treatment following the centrifugal casting intended to give the centrifuged tube a structure making it lighter.
After centrifugal casting and thermal treatment, tubes--that is to say cylindrical pipes of constant thickness--made from spheroidal graphite cast-iron at present have a ferritic structure which has two advantages: on the one hand, this structure gives them good mechanical characteristics (elastic resilience and ductility), on the other hand, this ferritic structure is easily obtained by thermal treatment after centrifugal casting, either in a chill-mould provided internally with a thick coating of a pulverulent mixture of silica and bentonite in suspension in water (a so called "wet-spray" coating) or in a chill-mould without such a coating.
In the case of the presence of a "wet-spray" coating on the chill-mould, the tube is extracted from its chill-mould and rapidly introduced into a furnace before it has cooled too much then is subject to a thermal treatment known as "maintaining ferritisation" at a temperature of the order of 750.degree. C., for a period of time of the order of 20 to 25 minutes, then it is left to cool naturally.
If there is no "wet-spray" coating on the chill-mould, the pipe is extracted from its casting mould and is introduced rapidly into a furnace where it is subjected to graphitisation annealing at a temperature of the order of 950.degree. C. for a period of time of the order of 20 to 25 minutes, then to maintaining ferritisation at a temperature of the order of 750.degree. C. for a period of time of the order of 15 to 20 minutes.
The Applicant has previously addressed the problem of obtaining economically cast-iron tubes produced by centrifugal casting, which are lighter than current tubes, without any appreciable loss of mechanical features.
The Applicant has sought to achieve this result by giving the tube of spheroidal graphite cast-iron a bainitic structure, instead of the customary ferritic structure, which bainitic structure has a tensile strength and characteristic of elongation as well as a characteristic of resilience equal to or greater than those of the ferritic structure.
The bainitic structure of the spheroidal cast-iron has already been studied for cast-iron parts cast in a chill-mould, in particular for mechanical parts of motor vehicles, as described for example in French Pat. No. 1 056 330, on account of the good mechanical characteristics conferred by a structure of this type.
In an article of the journal "Hommes et Fonderie" (Men and the Foundry) No. 84 of April 1978, a thermal treatment for obtaining this bainitic structure is described. The thermal treatment described is that of so called "chilling by stages" which makes it possible to lead to the bainitic structure by passing through austenitisation by successive stages of cooling at different speeds, of which one is chilling, starting with the hot part, as it has been cast. This treatment has the advantage of not requiring initial heating for austenitisation.
However, according to the technique described in this article, in view of the low aptitude for chilling of spheroidal graphite cast-iron, not only is it necessary to have very close checking of the carbon, silicon and manganese contents of the cast-iron, but also, if one wishes to treat relatively thick parts, it is necessary to add expensive alloying elements such as molybdenum, which even in small quantities are particularly effective for increasing the aptitude for chilling of cast-iron to a sufficient extent, in order that the chilling by stages prevents the formation of perlite and leads to the formation of bainite.